This K08 Career Development Award details a five-year training program to foster the career goal of Dr. Sang H. Min to become an independent investigator in megakaryocyte and platelet biology. Dr. Min is currently an Instructor in Medicine at the University of Pennsylvania. She completed a residency in Internal Medicine at Albert Einstein College of Medicine and a fellowship in Hematology-Oncology at the University of Pennsylvania. Recently, she also defended a Ph.D. in Cell and Molecular Biology at the University of Pennsylvania. During the course of the award period, Dr. Min will learn new scientific knowledge, acquire new skills, and carry out the proposed research and training under the mentorship of Drs. Charles S. Abrams and Mortimer Poncz. To enhance her scientific and professional development, an advisory committee of world- renowned physician-scientists in different disciplines has been assembled. The members of her advisory committee are Drs. Garret Fitzgerald, Lawrence F. Brass, and Mark L Kahn. The rich environment provided by the University of Pennsylvania has all the necessary facilities, equipment, expertise, and training for Dr. Min to complete her project successfully. The proposed research will focus on the role of platelet lysosomes in inflammation and thrombosis. Platelet secretory-products are believed to be crucial mediators in crosstalk between platelets and both endothelial and immune cells. However, the mechanisms by which this platelet- driven process leads to the development of inflammation and thrombosis are poorly understood. Dr. Min's long-term goal is to understand how platelets mediate cell interactions that promote inflammation and thrombosis, and identify potential targets for diagnosis and therapy. In a recently published work, Dr. Min demonstrated that platelet-specific inactivation of PIKfyve, a lipid kinase that modulates intracellular trafficing in mammals, disrupted the platelet lysosomal homeostasis and induced inflammatory and thrombotic responses in mice. This K08 proposal builds on this published work and proposes to understand normal role of PIKfyve in platelets, and how this lipid kinase contributes to a crosstalk between platelets and innate immune cells. The central hypothesis is that PIKfyve-null platelets release their lysosomes, which in turn induces tissue macrophages to drive inflammatory and thrombotic responses. The specific aims are: 1) Determine how PIKfyve regulates the platelet lysosomal homeostasis; 2) Define how PIKfyve-null platelets crosstalk with macrophages to promote inflammation; and 3) Determine how PIKfyve in platelets contributes to arterial thrombosis. Together, the results of this proposal are expected to provide mechanistic insights into how PIKfyve regulates platelet lysosomes as well as platelet-mediated communication with immune and vascular cells. These studies may identify potential targets for the diagnosis and therapy of platelet-mediated hemostatic and non-hemostatic diseases.